Steam generator

ABSTRACT

The invention relates to a steam generator (D 1,  D 2 ) having a first passage, which is formed by one or more pipes (R 1,  R 2 ) and through which a medium can be conducted in liquid and/or vapour form, and having a second passage (F 1,  F 2 ), through which a hot gas can be conducted along the outer side of each of the pipes (R 1,  R 2 ) which form the first passage. Each of the pipes (R 1,  R 2 ), which form the first passage, of the steam generator (D 1,  D 2 ) is surrounded by a pressure-resistant casing (M 1,  M 2 ) which has a heat-insulating coating (W 1,  W 2 ) on the inner side thereof and through the interior space of which the second passage (F 1,  F 2 ) runs.

The invention relates to a steam generator with a first passage that isformed by one or more tubes and through which a medium can be directedin liquid and/or vapor form, as well as a second passage through which afuel gas can be guided along the outer side of each of the tubes formingthe first passage.

Steam generators of the generic type have been state of the art for manyyears and are used, for example, as so-called water-tube boilers forgenerating steam in thermal power plants. A water-tube boiler has areaction chamber that is penetrated or bounded by water-carrying orsteam-carrying tubes the so-called water tubes. A carbon-containing fuelis usually burned in the reaction chamber at low underpressure. The heatthat is released during the combustion is taken up to a large extent bythe media that flow through the water tubes and is used for generatingand superheating steam. The superheated steam thus generated is thenexpanded in various pressure stages into a steam turbine connected to agenerator, whereby electrical current is generated.

The electrical efficiency of the above-described process is increasedwith the temperature and the pressure of the steam introduced into thesteam turbine. For this reason, it is desired to generate steam at thehighest possible pressure and the highest possible temperature and tofeed it to the steam turbine. According to the state of the art, it ispossible, with the assistance of a water-tube boiler, to generatehigh-pressure steam at a maximum of 600° C. and 280 bar (a) as well asmedium-pressure steam at a maximum of 620° C. and 60 bar (a). In orderto further increase the electrical efficiency, it is desired toconsiderably increase these values and to provide high-pressure steam at700° C. and 350 bar (a) as well as medium-pressure steam at 720° C. and60 to 120 bar (a). The main difficulty that is to be overcome in thisconnection consists in developing suitable materials from which watertubes can be manufactured at reasonable cost and which have asufficiently long service life under such extreme conditions. In theopinion of experts, this may take until the next decade.

The object of this invention is therefore to indicate a steam generatorof the above-described type, which can be manufactured from materialsthat can currently be obtained on the market and which neverthelessmakes it possible to generate steam at higher temperatures and pressuresthan that which is possible according to the state of the art.

This object is achieved according to the invention in that each of thetubes forming the first passage is surrounded by a pressure-resistantcasing, which has a heat-insulating coating on its inner side andthrough whose inner space the second passage runs.

A shell construction, which is configured and sized in such a way thatit is able, under operating conditions, to tolerate an internal pressurethat exceeds the external pressure by at least 1 bar, is to be providedunder a pressure-resistant casing.

Because of the pressure-resistant casing, it is possible to guide a fuelgas at a pressure that is elevated relative to the atmospheric pressurethrough the second passage. Since the pressure of the fuel gas acts onthe outer sides of the tubes forming the first passage, the pressure ofthe media guided into the interior of the tubes can be completely orpartially compensated and thus the mechanical stress of the tubes can beat least reduced. For this reason, it is possible to reduce the wallthickness of the tubes in comparison to the state of the art and/or tooperate the tubes at higher internal pressure and/or at highertemperatures. In addition, the transfer of heat to the tubes forming thefirst passage is improved by the elevated pressure of the fuel gas, bywhich it is possible to reduce the fuel gas temperature with the sameheat output.

Configurations of the steam generator according to the inventionspecifically call for a pressure-resistant casing or severalpressure-resistant casings, whereby each casing specifically surrounds atube or several tubes of the first passage.

Preferably, a pressure-resistant casing consists of steel, especiallypreferably a steel that is used according to the state of the art forthe construction of steam generators. The steel can be refractory steel.

Because of the heat-insulating coating, it is possible to reduce thethermal stress of the pressure-resistant casing to the extent that evennon-refractory steel can be reasonably used. In particular, when thecasing temperature in the operation of the steam generator can belimited to lower than 70° C., the use of fiber composite material isalso conceivable. In this case, a fiber composite material is defined asa material that usually comprises a plastic matrix, which is reinforcedby embedded fibers, such as, for example, glass or carbon fibers. Inaddition, a pressure-resistant casing, which consists of a combinationof steel and/or aluminum and/or a fiber composite material, is alsoconceivable.

A variant of the invention calls for a pressure-resistant casing to bedesigned as a tube. In this case, the cross-section of the tube can haveany shape, whereby it preferably has a circular shape, however.

Another variant of the invention calls for a pressure-resistant casingto be formed from flat or profiled sheets.

The first passage of a steam generator according to the invention canconsist of tubes of any shape. The first passage preferably comprises,however, at least one straight tube or one coiled tube.

A pressure-resistant casing of the steam generator according to theinvention is designed for an operating pressure of between 2 and 100 bar(a) and an operating temperature of between 60 and 350° C.

The heat-insulating coating consists of a material that ischemically-resistant and physically-resistant relative to the fuel gasesused in the operation of the steam generator according to the invention,in the case of which fuel gases these are primarily gases that aregenerated by combustion or gasification of carbon-containing feedstocks.The heat-insulating coating preferably consists of aluminum oxide(Al₂O₃) and/or zirconium oxide (ZrO₂) and/or silicon oxide (SiO₂) and/orsilicon carbide (SiC).

The evaporator according to the invention can be used to evaporate anyliquid and/or to superheat the steam thereof. Preferably, however, it issuitable for evaporating water and/or for superheating steam.

Below, the invention is to be explained in more detail based on twoembodiments depicted diagrammatically in FIGS. 1 and 2.

FIG. 1 shows the sectional view of a steam generator according to theinvention, whose first passage is formed by several tubes, whichtogether are surrounded by a pressure-resistant casing.

FIG. 2 shows the sectional view of a steam generator according to theinvention, whose first passage is formed by several tubes, of which eachindividual tube is surrounded by a pressure-resistant casing.

The steam generator D1 shown in FIG. 1 has tubes R1, which form a firstpassage, through which, for example, water and/or steam can be directedat a pressure of up to 350 bar (a) and at a temperature of up to 700° C.The tubes R1 are surrounded by a pressure-resistant casing M1 thatconsists of refractory steel, which is designed as a tube with a roundcross-section. In order to protect the pressure-resistant casing M1 fromexcessive thermal stress during the operation of the steam generator D1,a heat-insulating coating W1 is arranged on its inner side. The freespace F1 within the pressure-resistant casing M1 forms a second passage,through which fuel gas at an elevated pressure can be guided along theouter sides of the tubes R1.

The steam generator D2 that is shown in FIG. 2 has tubes R2, which forma first passage, through which, for example, water and/or steam can bedirected at a pressure of up to 350 bar (a) and at a temperature of upto 700° C. Each of the tubes R2 is surrounded by a pressure-resistantcasing M2 that consists of refractory steel, which is made of sectionalsheets. In order to protect the pressure-resistant casing M2 fromexcessive thermal stress during the operation of the steam generator D2,a heat-insulating coating W2 is arranged on its inner side. The freespaces F2 within the pressure-resistant casing M1 form a second passage,through which fuel gas at an elevated pressure can be guided along theouter sides of the tubes R2.

1. Steam generator with a first passage that is formed by one or moretubes and through which a medium can be directed in liquid and/or vaporform, as well as a second passage through which a fuel gas can be guidedalong the outer side of each of the tubes forming the first passage,characterized in that each of the tubes forming the first passage issurrounded by a pressure-resistant casing that has a heat-insulatingcoating on its inner side and through whose inner space the secondpassage runs.
 2. Steam generator according to claim 1, wherein itspecifically comprises a pressure-resistant casing or severalpressure-resistant casings, whereby a pressure-resistant casingsurrounds at least one tube of the first passage.
 3. Steam generatoraccording to claim 1, wherein a pressure-resistant casing consists ofsteel and/or aluminum and/or a fiber composite material.
 4. Steamgenerator according to claim 1, wherein a pressure-resistant casing isdesigned as a tube or is formed from flat or profiled sheets.
 5. Steamgenerator according to claim 1, wherein the first passage is designedwith one or more straight tubes.
 6. Steam generator according to claim1, wherein the first passage is designed with one or more coiled tubes.7. Steam generator according to claim 1, wherein a pressure-resistantcasing is designed for an operating pressure of between 2 and 100 bar(a) and an operating temperature of between 60 and 350° C.
 8. Steamgenerator according to claim 1, wherein the heat-insulating coatingconsists of aluminum oxide (Al₂O₃) and/or zirconium oxide (ZrO₂) and/orsilicon oxide (SiO₂) and/or silicon carbide (SiC).
 9. Steam generatoraccording to claim 1, wherein it is suitable for evaporating waterand/or for superheating steam.